Tennis player Serena Williams competes in the women's singles finals at the 2018 U.S. Open in Flushing NY on September 8, 2018. Naomi Osaka defeated Serena Williams. 08 Sep 2018 Pictured: (L-R) Tennis players Naomi Osaka hides her face while standing next to Serena Williams during the trophy presentation ceremony at the 2018 U.S. Open in Flushing NY on September 8, 2018. (Photo by Andrew Schwartz

After taking out the match in straight sets, 6-2, 6-4, Osaka and Williams appeared together for the trophy ceremony, during which audible boos erupted from the crowd.

In a raw moment, the 23-time Grand Slam singles winner reached over to comfort her young opponent and whisper something in her ear.

Serena Williams argues with the chair umpire during the US Open women’s final. Picture: Greg Allen/Invision/AP

Osaka, who was noticeably teary during the ceremony and at one point pulled her visor down over her eyes, appeared on The Ellen DeGeneres Show on Wednesday and cleared up exactly what her idol said to her.

"She said that she was proud of me and that I should know that the crowd wasn't booing at me," Osaka told DeGeneres of the emotional moment between rivals.

"At the time, I did … think that they were booing at me. I couldn't tell what was going on, because it was just so loud in there. It was a little bit stressful."

Reflecting on her incredible victory, Osaka said she always believed she had a chance to beat Williams.

"When you're little, you're taught not to look at, like, if your opponent gets angry or anything," Osaka said. "You're told to just turn around and try to focus, so I tried to do that, but in my mind I was sort of - like I really wanted to know what was going on."

She continued, "I couldn't hear and I was looking away, but I heard a lot of people in the crowd making noises, and I really wanted to turn around, but I didn't."

Osaka is already on Adidas's payroll, however is only reportedly making a "six-figure" salary that will expire this year.

If the monster contract goes through, it would be the biggest deal the apparel giant has ever given to a female tennis player and could propel Osaka up the ranks on Forbes' 2018 list of highest paid female athletes, which Williams currently leads.

In 2010, tennis star Maria Sharapova made history by signing the most lucrative female sports contract ever with Nike - a 10-year contract worth an estimated $97.6 million.